This invention relates to an interference filter of the type reflecting a certain wavelength band within a given wavelength range while letting pass other wavelength bands of the range.
Conventional interference filters of the above type are generally comprised of a plane surfaced substrate, such as glass, upon which has been deposited by e.g. vacuum deposition a plurality of alternate high and low refractive index thin uniform layers where each layer is arranged in a quarter-wave thickness or an odd multiple of a quarter-wave, at the chosen wavelength which is required to be reflected. Reflection efficiency and band width of the reflection zone are controlled by the ratio of the index of refraction of each high-low index layer pair. The efficiency of reflection can be increased by increasing the number of layer pairs.
Such conventional quarter-wave designs are in theory capable of providing narrow band reflection at specified points anywhere within a wide spectral range but in practice there is a limit to the choice of materials available for any particular wavelength band which it is required to reflect. Whilst in some parts of the electro-magnetic spectrum there is a wide choice of materials available, in other parts, such as in the infra-red region, there are only a very limited number of materials available. This can make the construction of narrow band reflection filters difficult and in some cases impractical.
It is an object of the present invention to provide an interference filter for narrow band reflection in which the aforementioned disadvantages are obviated.